This invention relates to a release mechanism and, more particularly but not exclusively, is concerned with a release mechanism for enabling a tool to be detached from a tool string during the construction, maintenance and repair of oil and gas wells.
During the construction, maintenance and repair of oil and gas wells it is frequently necessary to locate a tool at a given position in the wellbore. This is typically achieved by lowering the tool on the end of a tool string until the tool reaches the desired position. The tool is then secured in place. The tool is then separated from the tool string which is withdrawn from the wellbore.
A large number of release mechanisms are used to facilitate the separation of the tool string from the tool. One release mechanism is generally referred to as a "J-slot". In particular, after the tool has been secured in place the tool string is lowered by a small distance relative to the tool, rotated (typically through 30.degree.), and then raised to effect separation. Another release mechanism comprises a long coarse left hand threaded joint. Other release mechanisms include a valve seat. When it is desired to release the tool from the tool string an actuator such as a ball or dart is released down the wellbore. The actuator comes to rest on the valve seat. Pressure is then applied to the actuator and this is utilised to release the tool.
Whilst these release mechanisms are generally acceptable they occasionally fail which can cause serious delays. As a result of these failures it is now common for oil companies to require a contractor to provide a tool with a secondary release mechanism in case the primary release mechanism fails.
One secondary release mechanism which is commonly used comprises a shear pin which is not subject to stress when the tool string is rotated in its usual sense ("right hand torque") but which is subject to stress and can be sheared when the tool string is rotated in the opposite sense ("left hand torque"). The problem with this arrangement is that the shear pin can be inadvertently sheared during normal operations, for example if the tool is being rotated and the top drive stalls the torsional energy stored in the tool string may cause the tool string to spin anti-clockwise generating an inertia in the tool string which may cause the tool string to rotate anti-clockwise relative to the tool and sheer the shear pin. Furthermore, if the top drive stalls the inertia of the tool itself may cause the tool to rotate relative to the tool string and shear the shear pin. This problem is particularly acute when the tool is used for supporting a liner which needs to be rotated prior to cementing. In this case the inertia is influenced by the combined weight of the liner and tool.
Typically unplanned disconnections are caused by relatively intense forces of short duration.
The aim of the present invention is to provide a release mechanism which is less susceptible to release by such forces and which, whilst primarily intended as a secondary release mechanism could also be used as a primary release mechanism.